The price is good, but you get what you pay for.
Consider the following before you buy it/them:
1.5 Megapixel 1/5.8" CMOS - That's a REALLY small chip and will not perform well in low light conditions. It's so small it doesn't even show up on comparison charts. It will have an enormously deep depth of field (which is perfectly fine for your purposes. Most consumer cameras have large dof anyway.) A 1/3" sensor will have four times the surface area. (My Canon has a 1/2.7".)
It claims to be 1080i (60fps.) This is often code for 540p. With a 1.5 mp chip, I can't see how they can claim it will give you a 2.1 mp image. Unless they're processing the image by upscaling it like your TV does when it plays a DVD.
For someone who wants to shoot wedding videos for money I would stay away from this camera. I just don't think it will mesh well with your HMC150 (which I hear is a pretty solid performer. Kudos.) The SD40 might work for cut-aways of the audience in decent lighting but I wouldn't use it for anything beyond that.
I can understand wanting to stick with Panasonic (I'm a Canon guy, myself.) So consider buying one HDC-SD800 instead of two of the others. It has a slightly larger chip - three in fact. Low light performance is rated similarly however the three chips should provide better color and dynamic range under difficult conditions.
The HDC-SD800 is also a true HD camera that can shoot up to 1080p 60 instead of the 1080i 60.
Overall, it's got a lot more options and better guts. I think you'll be much happier in general.
Use one camera for your basic wide shot and the other close-ups and cutaways. You don't need two close-up cams on the couple. Use just one so you can see them look at each other at the same time. And shoot from either the center or from the groom's side towards the bride. She's the one you need to make happy. (Or her mom, usually.)
Consider putting the smaller cam in the loft shooting the wide, establishing shot. (Face the altar.) And stand on the groom's side with the nicer camera and change your shots periodically to get closeups and cutaways. Don't lose track of the ceremony though - you'll want to be on a tight shot for the key parts like the vows, the ring and kiss.
And one other consideration about placement - everyone stands up when the bride comes down the aisle so don't get caught with a great shot that gets blocked by the crowd.
Regarding your second camera, and this is purely marketing, accessorize it to make it LOOK more professional. Most clients don't know anything about cameras. They think if it's bigger it's better. Throw on a lens hood for about 5-10 bucks to help build the perception of a professional camera and the quality that comes with it.
I recently had my new Canon HD camera next to my XL1 and everyone commented on what a great camera the XL1 must be. Little did they know the small black camera at its side had better video. The mere appearance of my XL1 used to get me a lot of referals.
Another accesory to consider is a wide angle attachment. It's sometimes hard to get a wide enough angle with a consumer camera if you put it up close. Avoid something that will cause barrel distortion, however. Only skateboarders and skiers like to see that in their videos.
You may also want to add a larger battery and another memory card. Amazon has Transcend cards that are very affordable and off brand batteries WAY cheaper than OEM.
Regarding audio - place the mic on the officiant, not the groom. It will be evenly centered between bride and groom. And once the groom hits that aisle you'll spend the next hour trying to track down your mic. He (and the new Mrs.)certainly doesn't want to be distracted from the moment by getting it back to you.
I hope this has helped.
